High-angle Annular Dark-field Imaging Research Articles

Interface tomography of GaInAs/AlInAs quantum cascade laser active regions

We present a high-resolution electron tomography study of buried ultra-thin layers and their interfaces from the active region of a (Ga,In)As/(Al,In)As quantum cascade laser (QCL) test structure. Using a high-angle annular dark-field scanning transmission electron microscopy image series, a three-dimensional (3D) reconstruction of the complex layer structure is obtained. From this 3D information, we determine quantitative values for the chemical width and, simultaneously and independently, the root mean square roughness (rms) and lateral correlation length of the individual interfaces of a cascade using topographic height maps. The interfacial widths are comparably small for all interfaces within a cascade and the layer thicknesses show only a small standard deviation of less than one monolayer. The rms roughness is systematically lower at the direct (Ga,In)As-on-(Al,In)As interface compared to its inverse. In addition, using the one-dimensional height–height correlation function, different lateral correlation lengths along the two perpendicular in-plane directions are detected indicating a distinct anisotropy of the interface morphology. Our accurate and comprehensive results on the QCL test structure will serve as feedback to evaluate the growth process and help to assess the performance of corresponding future laser devices in detail.

Polarity determination of crystal defects in zincblende GaN by aberration-corrected electron microscopy

Aberration-corrected scanning transmission electron microscopy techniques are used to study the bonding configuration between gallium cations and nitrogen anions at defects in metalorganic vapor-phase epitaxy-grown cubic zincblende GaN on vicinal (001) 3C-SiC/Si. By combining high-angle annular dark-field and annular bright-field imaging, the orientation and bond polarity of planar defects, such as stacking faults and wurtzite inclusions, were identified. It is found that the substrate miscut direction toward one of the 3C-SiC ⟨110⟩ in-plane directions is correlated with the crystallographic [1–10] in-plane direction and that the {111} planes with a zone axis parallel to the miscut have a Ga-polar character, whereas the {111} planes in the zone perpendicular to the miscut direction have N-polarity. The polarity of {111}-type stacking faults is maintained in the former case by rotating the coordination of Ga atoms by 180° around the ⟨111⟩ polar axes and in the latter case by a similar rotation of the coordination of the N atoms. The presence of small amounts of the hexagonal wurtzite phase on Ga-polar {111} planes and their total absence on N-polar {111} planes is tentatively explained by the preferential growth of wurtzite GaN in the [0001] Ga-polar direction under non-optimized growth conditions.

Fundamental Understanding of the Effect of a Polyaniline Coating Layer on Cation Mixing and Chemical States of LiNi0.9Co0.085Mn0.015O2 for Li-Ion Batteries

A high nickel content of the cathode usually results in a large discharge capacity but causes structural collapse. Ni2+ ions move to the Li layer when Li+ ions are deintercalated during discharge, resulting in irreversible phase transition, cation mixing, dissolution of transition metal ions, and side reactions. A protective barrier is essential for maintaining the layered structures of cathode materials, even after several charge/discharge cycles of Li-ion batteries. Polyaniline (PANi) is an organic coating material with high conductivity and flexibility. PANi-coated cathodes have been widely reported for improving electrochemical performances. However, it is insufficient to prove the correlation between the PANi coating layer and structural stability through further analysis after an electrochemical test. Therefore, we focused on the structural stability and chemical states of the PANi-coated cathode after a cycle test by observing the morphology, lattice patterns, and chemical states of the surface. PANi-coated LiNi0.9Co0.085Mn0.015O2 (NCM; PANi@NCM) exhibited an initial discharge capacity of 221 mAh g–1 and a capacity retention of 81% after 50 cycles at 45 °C, which corresponded to an improved performance compared to pristine NCM. The cycled PANi@NCM showed an identical morphology to that of the cathode before the test. The R3̅m layered structure of PANi@NCM was maintained even after 50 cycles, as confirmed by transmission electron microscopy analysis with fast Fourier transform patterns and high-angle annular dark-field images. In addition, PANi@NCM maintains a thinner passivation layer (8 nm) compared with that of pristine NCM (27 nm). According to the X-ray photoelectron spectroscopy results, the surface chemical state of PANi@NCM showed that side reactions between the cathode and the electrolyte were suppressed during the cycle test. Therefore, it is demonstrated that the PANi coating layer prevents cation mixing and side reactions.

Novel strategy for space group determination in real space

Traditional space group determination methods are all in reciprocal space, which involves ambiguous identification on some space groups which have glide plane and screw axes. The novel strategy herein for space group determination in real space is based on the atom resolution high angle annular dark field (HAADF) technology. Three HAADF images in three specific crystal zone axes are needed at most. The proposed strategy for space group determination is easy and effective.

Suzuki hardening and segregation in Co0.95Cr0.8Fe0.25Ni1.8Mo0.475 high-entropy alloys

A Co0.95Cr0.8Fe0.25Ni1.8Mo0.475 high-entropy alloy was heavily cold-rolled and subsequently aged at 773 K. Aging-induced Suzuki hardening was observed. High-angle annular dark-field scanning transmission electron microscopy imaging and the corresponding energy dispersive X-ray spectroscopy elemental distribution mapping suggested the enrichment of Co atoms at stacking faults (SFs), which pinned down partial dislocations and enhanced the resistance of the alloy to plastic deformation. Furthermore, the enrichment of Co and Mo atoms at SFs was observed in a low dislocated, strain-aged specimen. The significant difference in the SF density of the specimens led to difference in segregation. The local stacking fault energy (SFE) with the segregation of Co and Mo atoms was calculated as −30.6 mJ/m2 at the aging temperature. The negative SFE did not restrict the SF expansion, indirectly resulting in the formation of wide SFs.

Model-based deconvolution for particle analysis applied to a through-focus series of HAADF-STEM images.

This paper presents an approach for determining the sizes and three-dimensional (3D) positions of nanoparticles from a through-focus series of high-angle annular dark-field scanning transmission electron microscopy images. By assuming spherical particles with uniform density, the sizes and 3D positions can be derived via Wiener deconvolution using a series of kernels prepared by the convolution of the 3D point spread function of the electron beam and the 3D density distribution of spheres with different radii. This process is referred to as a model-based deconvolution. Four 3D datasets with a volume size of 148 × 148 × 560 nm3 were obtained from the four sets of 256 high-angle annular dark-field scanning transmission electron microscopy images of 256 × 256 pixels taken from the same field of view under the through-focus condition. The 3D positions and radii of 14 particles in each 3D dataset were derived using the model-based deconvolution for ∼8 min. The observation errors of the 3D position were estimated as σx ≅ σy ≅ 0.3 nm and σz < 1.6 nm.

Statistical Determination of Atomic-Scale Characteristics of Au Nanocrystals Based on Correlative Multiscale Transmission Electron Microscopy

Abstract Atomic-scale characteristics of individual nanocrystals (NCs), such as the crystallographic orientation of their facets and the kind and density of crystal structure defects, play a tremendous role for the functionality and performance of the whole NC population. However, these features are usually quantified only for a small number of individual particles, and thus with limited statistical relevance. In the present work, we developed the multiscale approach available in transmission electron microscopy (TEM) further, and applied it to describe features of different types of Au NCs in a statistical and scale-bridging manner. This approach combines high-resolution TEM, which is capable of describing the characteristics of NCs on atomic scale, with a semi-automatic analysis of low-magnification high-angle annular dark-field scanning TEM images, which reveals the nanoscopic morphological attributes of NCs with good statistics. The results of these complementary techniques are combined and correlated. The potential of this multiscale approach is illustrated on two examples. In the first one, the habitus of Au NCs was classified and assigned to multiply twinned nanoparticles and nanoplates. These classes were quantified and related to different stacking fault densities. The second example demonstrates the statistical determination of crystallographic orientations and configurations of facets in Au nanorods.

Structural Evolution in an Annealed (Eu, Tb)-Doped ZnO/Si Nanoscale Junction: Implication for Red LED Development

Codoped (Tb,Eu) ZnO films grown by magnetron sputtering on a silicon substrate and annealed up to 1200 °C showed intense photoluminescence (PL) emission from Eu3+ ions. The high-temperature annealing led to diffusion and segregation of rare earth (RE) elements toward the bottom of the film, which induced the formation of nanometric Zn-free inclusions responsible for remarkable PL emission intensity. Combined electron diffraction, chemical contrast imaging, and optical studies of these nanometric phases have been carried out. Large inclusions of zinc silicates and RE silicates a few hundreds of nanometers in size were observed, embedded in a silica phase. The structural determination of these RE-rich inclusions was carried out by combining atomic Z contrast imaging (high-angle annular dark-field imaging) and precession electron diffraction data. Upon annealing at 1200 °C, it appeared that the structure was related to an F-type disilicate structure. Energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy experiments were carried out to determine the ratios between elements and the oxidation states of the RE elements in the abovementioned inclusions. A (Tb,Eu)2Si2O7 formulation is proposed from dynamical precession electron diffraction tomography refinements, leading to a +III valence state for the RE species in agreement with spectroscopic results. PL modeling is also in good agreement with the experimental data. These results complete those obtained at 1100 °C for which the inclusions were identified as some RE10–x(SiO4)6O2–x oxyapatite structures and pointed out a combined structural and PL properties’ evolution between 1100 and 1200 °C annealing temperatures.

Atomic-Scale Dynamics at Solid–Liquid Nanointerfaces Induced by Electron-Beam Irradiation

We report electron-beam induced dynamics at PbS-Pb solid-liquid interfaces. We fabricated PbS-Pb solid-liquid nanointerfaces by heating PbS nanocrystals under vacuum to observe the entire process of structural evolution at the atomic scale. We investigated the dynamics using time-resolved high-angle annular dark-field imaging. Electron-beam irradiation caused layer-by-layer dissolution of PbS at the interface, resulting in the formation of the Pb nanodroplet. Ordered liquid layers were observed adjacent to the interface even under continuous electron-beam irradiation and followed the movement of the interface. Instantaneous epitaxial growth of PbS was observed as a reverse process of the dissolution. The resultant Pb nanodroplet provides indisputable evidence for selective sputtering of sulfur atoms via electron-beam irradiation. This paper demonstrates atomic resolution in situ observations of selective and complete sputtering. The observed dynamics can be explained by the intermittent phase transition via nonequilibrium states of the solid-liquid nanointerface triggered by selective sputtering.

Improving data quality for three-dimensional electron diffraction by a post-column energy filter and a new crystal tracking method.

Three-dimensional electron diffraction (3D ED) has become an effective technique to determine the structures of submicrometre- (nanometre-)sized crystals. In this work, energy-filtered 3D ED was implemented using a post-column energy filter in both STEM mode and TEM mode [(S)TEM denoting (scanning) transmission electron microscope]. The setups for performing energy-filtered 3D ED on a Gatan imaging filter are described. The technique and protocol improve the accessibility of energy-filtered 3D ED post-column energy filters, which are available in many TEM laboratories. In addition, a crystal tracking method in STEM mode using high-angle annular dark-field imaging is proposed. This method enables the user to monitor the crystal position while collecting 3D ED data at the same time, allowing a larger tilt range without foregoing any diffraction frames or imposing extra electron dose. In order to compare the differences between energy-filtered and unfiltered 3D ED data sets, three well known crystallized inorganic samples have been studied in detail. For these samples, the final R 1 values improved by 10-30% for the energy-filtered data sets compared with the unfiltered data sets, and the structures became more chemically reasonable. Possible reasons for improvement are also discussed.

Different atomic contrasts in HAADF images and EELS maps of rutile TiO2.

High-angle annular dark-field (HAADF) imaging and elemental mapping at the atomic scale by scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) are widely used for material characterization, in which quantitative understanding of the contrast of the image is required. Here, we report an unexpected image contrast in the elemental mapping of rutile TiO2, where the Ti L2,3 map shows an anisotropic elliptical shape that extends along the long axis in the octahedral structure, while the atomic contrast of Ti columns in the HAADF image is almost circular. Multi-slice simulation reveals that unique electron channeling related to the rutile structure and the difference of the potentials between HAADF and EELS cause the different atomic contrasts in the two images.

Large refrigerant capacity in superparamagnetic iron nanoparticles embedded in a thin film matrix

A magnetocaloric effect (MCE) with sizable isothermal entropy change (ΔS) maintained over a broad range of temperatures above the blocking temperature is reported for a rare earth-free superparamagnetic nanoparticle system comprising of Fe–TiN heterostructure. Superparamagnetic iron (Fe) particles were embedded in a titanium nitride (TiN) thin film matrix in a TiN/Fe/TiN multilayered pattern using a pulsed laser deposition method. High angle annular dark-field images in conjunction with dispersive energy analysis, recorded using scanning transmission electron microscopy, show a clear presence of alternating layers of Fe and TiN with a distinct atomic number contrast between Fe particles and TiN. Quantitative information about the isothermal entropy change (ΔS) and the magnetocaloric effect in the multilayer Fe–TiN system has been obtained by applying Maxwell relation to the magnetization vs temperature data at various fields. With the absence of a dynamic magnetic hysteresis above the blocking temperature, the negative ΔS as high as 4.18 × 103 J/Km3 (normal or forward MCE) is obtained at 3 T at 300 K.

Effects of flexoelectric polarization on surface potential of dielectric thin-film heterostructures: A comparative study

Recently, flexoelectric effect has attracted considerable attention owing to ubiquitous existence in all dielectrics, regardless of the symmetry. It promises intriguingly physical phenomena, such as strain gradient-induced electric polarizations, photocurrents, and interfacial transports, as well as their electromechanical coupling with external force loading, in diverse materials for multifunctional applications in electronics. In this work, we report the flexoelectric-modulation on surface potential of LaFeO3 (LFO) thin-film heterostructures. The LFO thin film with or without the flexoelectric effect has been achieved by controlling epitaxial misfit against a substrate. Lattice structures and strain behaviors are observed by atomic-resolution high-angle annular dark-field imaging. Grown on a LaAlO3 substrate, a giant strain gradient of ∼3 × 106 m−1 is generated in the LFO thin film due to the gradual relaxation of large misfit strain with increasing thickness, yielding a robust flexoelectric polarization pointing to the heterostructure surface. In contrast, the LFO is almost fully strained on a SrTiO3 substrate due to the small lattice mismatch. The flexoelectric polarization results in an increase in surface potential in the LFO heterostructure due to the incomplete screening of positive polarization bound charges, as observed by scanning kelvin probe microscopy. Furthermore, x-ray photoelectron spectroscopy reveals that the flexoelectric polarization can downward bend the band alignment of the LFO layer and modulate the interfacial potential barriers. These results provide the way for experimental observations of the flexoelectric effect and deliver physical insight into deep understanding of interfacial electronic structures of flexoelectric-based devices.

Selective area heteroepitaxy of InAs nanostructures on nanopillar-patterned GaAs(111)A

A process sequence enabling the large-area fabrication of nanopillar-patterned semiconductor templates for selective-area heteroepitaxy is developed. Herein, the nanopillar tops surrounded by a SiNx mask film serve as nanoscale growth areas. The molecular beam epitaxial growth of InAs on such patterned GaAs(111)A templates is investigated by means of electron microscopy. It is found that defect-free nanoscale InAs islands grow selectively on the nanopillar tops at a substrate temperature of 425 °C. High-angle annular dark-field scanning transmission electron microscopy imaging reveals that for a growth temperature of 400 °C, the InAs islands show a tendency to form wurtzite phase arms extending along the lateral ⟨112¯⟩ directions from the central zinc blende region of the islands. This is ascribed to a temporary self-catalyzed vapor–liquid–solid growth on {111¯} B facets, which leads to a kinetically induced preference for the nucleation of the wurtzite phase driven by the local, instantaneous V/III ratio, and to a concomitant reduction of surface energy of the nanoscale diameter arms.

Microscopic Insights into the Inhomogeneous Degradation of Pt Alloy Catalysts in PEM Fuel Cells

Fuel cell electric vehicles (FCEVs) powered by proton exchange membrane fuel cells (PEMFCs) offer a sustainable, low-carbon alternative to the world’s current energy structure based on fossil fuels. Recently, application of PEMFC to heavy-duty vehicles (HDVs) has attracted significant interest for reducing greenhouse gas emissions in this market. Relative to passenger vehicles, HDVs require extended lifetimes and high operating costs, leading to an increased emphasis on the durability and efficiency of membrane electrode assembly components [1].Degradation of the oxygen reduction reaction (ORR) cathode catalyst is one of the main challenges for heavy duty applications. Several degradation mechanisms have been identified, such as Ostwald ripening, particle coalescence/migration, and particle detachment, the observation of which relies on transmission electron microscopy (TEM) [2,3]. Through TEM, inhomogeneous degradation at the micro- and nanoscale during operation have been observed: (1) the Pt loading decreases toward the cathode/membrane interface and creates a depletion zone adjacent to the interface [2,4] and (2) different rates of electrochemical surface area loss of PtCo supported on various porous carbons,[5] leading to the speculation of different degradation rates for particles at the exterior vs. interior carbon surfaces.To elucidate these inhomogeneous degradation mechanisms, we employ high-throughput image analysis and electron tomography methods to analyze fuel cell cathodes before and after accelerated stress tests (ASTs) performed in both nitrogen and air environments. An automated, high-throughput method was developed that provides statistically relevant analysis of PtCo particle size, shape, and loading distribution. This method combines automated image or energy-dispersive spectroscopy (EDS) map acquisition using a commercial software (MAPS, Thermo Fisher Scientific) and custom Python codes to analyze high-angle annular dark field (HAADF) and EDS spectrum images. With the high-throughput method, we reveal the inhomogeneous distribution of PtCo particle size, Pt loading, and Co content across the entire electrode. In addition, an electron tomography workflow is under development to measure PtCo particle size and composition on the exterior and interior of the porous carbon support. The effect of cathode gaseous environment on the distribution of particle size, loading, and composition is discussed at the microscale across the electrode and at the nanoscale for exterior and interior surfaces of primary carbon particles. Findings from this study will provide new mitigation strategies and design principles to enable durable fuel cell catalysts.[6]References[1] DA Cullen et al., Nat. Energy. 6 (2021), p. 462.[2] PJ Ferreira et al., J. Electrochem. Soc. 152 (2005), p. A2256.[3] R Borup et al., Chem. Rev. 107 (2007), p. 3904.[4] H Yu et al., Electrochim. Acta 247 (2017), p. 1169.[5] M Ko et al., J. Electrochem. Soc. 168 (2021), p. 24512.[6] This material is based on work performed by the Million Mile Fuel Cell Truck (M2FCT) Consortium, technology managers Greg Kleen and Dimitrios Papageorgopoulus, which is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, and by U.S. Department of Energy, Office of Nuclear Energy, Fuel Cycle R&D Program, and the Nuclear Science User Facilities. Electron microscopy research was supported by the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Quantifying the Structure and Chemistry of Semiconductor Devices with the Latest Advancements in Scanning Transmission Electron Microscopy

Electron microscopy has been revolutionized by the advent of the aberration corrector and advances in detector both for imaging and spectroscopy. Aberration correction dramatically improved spatial resolution into the sub-ångstrom regime, while new detectors have unlocked information about materials that was previously only imaginable.While these recent advances have proven essential to the atomic scale characterization of materials, many measurements have remained largely semi-quantitative. In particular, accuracy and precision for scanning transmission electron microscopy (STEM) was significantly hampered by the presence of sample drift and scan distortion. Until recently, this limitation has obscured the capabilities to characterize minute changes to the atomic structure that can ultimately define material properties.In this talk, we will introduce revolving scanning transmission electron microscopy (RevSTEM) for the characterization of semiconductors. The method uses a series of fast-acquisition STEM images, but with the scan coordinates rotated between successive frames. This scan rotation introduces a concomitant change in image distortion that is then used to analyze the sample drift rate and direction. Multiple case studies will be presented to demonstrate the power of this technique to characterize electronic materials. For example, we will show how picometer level precision and accuracy enabled the capability to directly quantify ferroelectric HfO2 thin films. We will also show how RevSTEM images can be used to accurately determine crystallographic parameters in real-space, and to determine the structural origins of polarization.When combined with modern spectroscopy tools, the technique can directly inform growth mechanisms in AlN based materials for fabrication of quantum well structures. Specifically, We will show the interface quality between various AlGaN/AlN quantum wells using a combination of high-angle annular dark-field (HAADF) imaging and energy dispersive X-ray spectroscopy (EDS). We will show that we can directly measure the lattice parameters for each layer and how they vary across each AlN/AlGaN interface. Using EDS, we correlate the lattice parameters and lattice parameter values to the chemical composition in each layer. We show that throughout multiple quantum well structures, there is an asymmetrical interface with a diffusion of Ga on one side of the AlGaN layer. Further, we will discuss fluctuation in the composition present throughout each layer and the role on the local strain distribution.

Atomic-Scale Strucural Analysis of Pt-Based Nanoparticles Using Scanning Transmission Electron Microscopy

Polymer electrolyte fuel cells (PEFCs) is proposed as an alternative to the power generation system in fuel cell vehicles. The performance of the PEFCs is significantly affected by the catalytic activity of the cathode, which contains platinum in general. Recent theoretical studies show that the Pt-Pt interatomic distance at the subsurface of the Pt-based catalysts might considerably influence the catalytic activity. However, experimental evidence remains a challenge due to the requirement of measurement accuracy in picometers at the subsurface. The present work attempts to achieve precise measurements on the Pt-Pt interatomic distance of Pt particles by high-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy (STEM). Further, we also aim to understand the effects of the change in Pt-Pt interatomic distance on the catalytic activity.A catalyst of 10 wt.% Pt-loaded on graphitized carbon (Sigma-Aldrich) was used in this work. TEM specimen was prepared by dispersing the catalyst on a carbon film supported copper grid. Structural characterizations were examined by a spherical aberration corrected scanning transmission electron microscope (JEM-2100F, JEOL) operated at 200 kV.In order to determine the Pt atomic coordinate accurately, images were first taken under the optimized conditions. Further, a series of images were taken and integrated to form one image with high signal-to-noise ratio and minimized drifting. Finally, the Pt atomic coordinates were extracted precisely on a picometer scale by fitting of the individual atomic columns to two-dimensional Gaussian functions using a custom code developed by MatLab software. To gain insight into the atomic changes in Pt particle, a map of Pt-Pt interatomic displacement vector was constructed and superimposed on the HAADF-STEM image. Intriguingly, a significant Pt-Pt interatomic displacement occurs at the surface of the Pt particle, corresponding to changes in surface morphology and particle size.The state-of-the-art methodology demonstrated here could greatly advance our understanding of surface chemistry of Pt-based catalysts such as Pt3Co alloys. Further understanding of the correlation between the interatomic distances on the catalyst surface and the catalytic activity may give considerable interests and guidelines for future developments of PEFCs.

Rhodium nanocrystals on porous graphdiyne for electrocatalytic hydrogen evolution from saline water

The realization of the efficient hydrogen conversion with large current densities at low overpotentials represents the development trend of this field. Here we report the atomic active sites tailoring through a facile synthetic method to yield well-defined Rhodium nanocrystals in aqueous solution using formic acid as the reducing agent and graphdiyne as the stabilizing support. High-resolution high-angle annular dark-field scanning-transmission electron microscopy images show the high-density atomic steps on the faces of hexahedral Rh nanocrystals. Experimental results reveal the formation of stable sp–C~Rh bonds can stabilize Rh nanocrystals and further improve charge transfer ability in the system. Experimental and density functional theory calculation results solidly demonstrate the exposed high active stepped surfaces and various metal atomic sites affect the electronic structure of the catalyst to reduce the overpotential resulting in the large-current hydrogen production from saline water. This exciting result demonstrates unmatched electrocatalytic performance and highly stable saline water electrolysis.

Effect of Ti-Hf-Zr-Cu-Ni high entropy alloy addition on laser beam welded joint of Ti2AlNb based intermetallic alloy

• In this study, LBW of Ti-22Al-27Nb was experimented through addition of an interlayer of the filler, Ti-Hf-Zr-Cu-Ni HEA, into the joint and a comparison between the LBWJs of Ti-22Al-27Nb either prepared with or without addition of the filler was established through microstructural analysis by corelating its impact on mechanical properties at room temperature. Followings are the highlights of this experimental work. • The addition of Ti-Hf-Zr-Cu-Ni HEA into FZ of LBWJ of Ti-22Al-27Nb activated heterogenous nucleation due to difference of chemical composition between the BM and the filler, resultantly, fine grained B2 phase with greater fraction of HAGBs was evolved in the FZ. Grain refinement and higher misorientation angle between GBs in the FZ contributed an increase in hardness, tensile strength and percentage elongation of HEA adulterated LBWJ of Ti-22Al-27Nb. • The average hardness of the FZ (343.5 HV) and UTS (1062 MPa) of the HEA added LBWJ was found in close agreement with that of BM, 345 HV and 1060 MPa, respectively. • The percentage elongation of the joint, prepared with an interlayer of HEA, was measured up to 11.2%, which was 83.6% of the BM. • The LBWJ of Ti-22Al-27Nb, prepared with the addition of HEA, fractured from the junction of FZ and HAZ in a ductile mode while the LBWJ prepared without addition of Ti-Hf-Zr-Cu-Ni filler was ruptured from FZ in quasi-cleavage mode. Ti 2 AlNb based intermetallic alloys, a potential competitor for next-generation super alloys, are susceptible to high-temperature embrittlement due to nucleation of a metastable single B2 phase in the fusion zone (FZ) during laser beam welding (LBW). In this study, a high entropy alloy (HEA), Ti-Hf-Zr-Cu-Ni, was self-developed and introduced as an interlayer into laser beam welded joint (LBWJ) of Ti-22Al-27Nb to analyze its impact on the evolution of microstructure in the weld zone (WZ) and subsequently on joint performance. Microstructural examination was carried out through electron probe micro analysis (EPMA), electron backscattered diffraction (EBSD) analysis, high-resolution scanning transmission electron microscopy (HRSTEM) comprising bright field (BF), selective area electron diffraction (SAED) and high angle annular dark-field (HAADF) imaging. Addition of the HEA into FZ of LBWJ triggered heterogenous nucleation during solidification, resultantly, fine-grained B2 with a greater proportion of high angle grain boundaries (HAGBs) was developed. FZ of Ti-22Al-27Nb LBWJ, prepared with an interlayer of HEA, was composed of planar, cellular, columnar and equiaxed dendritic grains; a solidification mode which was different from that observed in LBWJ prepared without adulteration of the HEA. The impact of heterogenous nucleation during epitaxial solidification on mechanical properties was established through micro vickers hardness mapping and tensile test, conducted at room temperature. The average hardness, 343.5 HV, in the FZ of LBWJ prepared with an interlayer of HEA, was compatible with that of base material (BM), 345 HV. The ultimate tensile strength (UTS), 1062 MPa, and percentage elongation, 11.2%, of the HEA tempered LBWJ were found in close approximation with that of BM, 1060 MPa and 13.4%, respectively. A ductile mode of failure was observed during tensile test of the Ti-Hf-Zr-Cu-Ni supplemented LBWJ of Ti-22Al-27Nb, while quasi-cleavage mode of fracture was apparent in the joint of Ti-22Al-27Nb welded without addition of the HEA.

Analysis of three-dimensional chromatin packing domains by chromatin scanning transmission electron microscopy (ChromSTEM)

Chromatin organization over multiple length scales plays a critical role in the regulation of transcription. Deciphering the interplay of these processes requires high-resolution, three-dimensional, quantitative imaging of chromatin structure in vitro. Herein, we introduce ChromSTEM, a method that utilizes high-angle annular dark-field imaging and tomography in scanning transmission electron microscopy combined with DNA-specific staining for electron microscopy. We utilized ChromSTEM for an in-depth quantification of 3D chromatin conformation with high spatial resolution and contrast, allowing for characterization of higher-order chromatin structure almost down to the level of the DNA base pair. Employing mass scaling analysis on ChromSTEM mass density tomograms, we observed that chromatin forms spatially well-defined higher-order domains, around 80 nm in radius. Within domains, chromatin exhibits a polymeric fractal-like behavior and a radially decreasing mass-density from the center to the periphery. Unlike other nanoimaging and analysis techniques, we demonstrate that our unique combination of this high-resolution imaging technique with polymer physics-based analysis enables us to (i) investigate the chromatin conformation within packing domains and (ii) quantify statistical descriptors of chromatin structure that are relevant to transcription. We observe that packing domains have heterogeneous morphological properties even within the same cell line, underlying the potential role of statistical chromatin packing in regulating gene expression within eukaryotic nuclei.